No search was conducted on the subject matter of this specification in the U.S. Patent and Trademark Office or in any other search facility.
The most relevant prior art of which we are aware is U.S. Pat. No. 4,102,698, issued July 25, 1978 to Lange et al. This patent is for silicon nitride compositions in the Si.sub.3 N.sub.4 /Y.sub.2 O.sub.3 /SiO.sub.2 system. The patent discusses the ternary phase diagram of the Si.sub.3 N.sub.4, SiO.sub.2 and Y.sub.2 O.sub.3 system. The patent teaches that within that system there is a so-called stable range of compositions for materials and an unstable region for certain compositions of such materials. By unstable, it is meant that a material will oxidize and degrade at 1000.degree. C. under oxidizing conditions. If a material is stable, it does not oxidize under oxidizing conditions at 1000.degree. C.
W. D. Caruthers, in "3500-Hour Durability Testing of Ceramic Materials" NASA Contract DEN 3-27, Third Progress Report, December, 1978, observed that the Si.sub.3 N.sub.4 /Y.sub.2 O.sub.3 components defined as unstable by Lange are unstable in the temperature range of 700.degree.-1000.degree. C.
In a general sense, degradation of the material will result in strength losses after oxidation exposures. This is the ultimate test for a material. A stable material will have low oxidation weight gains, but more importantly will exhibit little or no strength losses after oxidation exposure.
Normally, the materials discussed in the Lange patent are materials in which generally individual pure grains of silicon nitride have grain boundaries formed of mixtures of silicon nitride, silicon dioxide and yttrium oxide. The yttrium oxide is introduced into these materials as a sintering aid and is usually present in a range of 3-10 mole%. The silicon dioxide is introduced into these materials as an impurity and remains as silicon dioxide no matter how carefully the silicon nitride powder is prepared.
We have found that we can make thermally stable ceramic materials even though the material in the grain boundary falls into materials classified by the Lange et al patent as being unstable. We have found further that we can make thermally stable materials if the secondary phase forming the grain boundaries is a substantially pure material rather than a mixture of various materials.